Microwave signal attenuator

ABSTRACT

A microwave signal attenuator includes at least two diodes shunting to ground points separated by a quarter of a centerfrequency wavelength on a transmission line. At a quarter centerfrequency wavelength from each diode toward the input and output ports respectively, are connected first and second circuits respectively, comprising an impedance and a diode in series. Bias current is fed via each circuit, the transmission line and the two diodes to ground to controllably vary the resistance of the diodes in accordance with the desired attenuation.

United States Patent [191 Sly Nov. 27, 1973 MICROWAVE SIGNAL ATTENUATOR[75 Inventor: Thomas L. Sly, Clay, N.Y. Pr'mary Gensler Attorneyl-laneet al. [73] Assignee: Anaren Microwave, Incorporated,

Syracuse, N.Y. 57 S C [22] Filed: Jan. 12, 1973 A microwave signalattenuator includes at least two diodes shunting to ground pointsseparated by a quar- [21] Appl' 322914 ter of a center-frequencywavelength on a transmission line. At a quarter center-frequencywavelength [52] US. Cl. 333/81 A, 323/81 from each diode toward theinput and output ports re- [51] Int; Cl. H0lp 1/22 spectively, areconnected first and second circuits re- Field of Search 333/3 R, 81 8spectively, comprising an impedance and a diode in 323/74, 80, 81series. Bias current is fed via each circuit, the transmission line andthe two diodes to ground to controlla- [56] References Cited bly varythe resistance of the diodes in accordance UNITED STATES PATENTS withthe desired attenuation. 3,135,934 6/1964 Schoenike 333/81 R 7 Claims, 1Drawing Figure 3,325,754 6/1967 Frisch et al. 333/81 R 3,453,564 7/1969Russell..... 33 3/8IR 3,713,037 l/l973 Hopfer 333/81 A /8 I r 1 20 I r2622 24 28 souRcE /6 SlNK OPEN 7' T OPEN ENDED ENDED LINE I 42 ewe:

BIAS 40 SOURCE MICROWAVE SIGNAL ATTEN UAT'OR /Z PATENTEDNUVEYIQB3.775.708

3a 4 6 OPEN l PEN EN ENDED 27:5 BIAS souRcE MICROWAVE SIGNAL ATTENUATOR/Z 1 MICROWAVE SIGNAL ATTENUATOR This invention pertains to microwavesignal attenuators and more particularly to absorptive shuntattenuators.

- Microwave signal attenuators are used to control the amplitude of amicrowave signal transmitted between, say, a source and a sink. Thesimplest of such attenuators is merely a PIN diode connected between themicrowave signal conductor (a transmission line, stripline or microstripline) and ground. In addition to feeding the microwave signal down theconductor, a direct current is also fed down the conductor. When thiscurrent passes through the diode, the shunt resistance of the diodebecomes smaller and some of the microwave energy is shunt to ground anddoes notreach the output port connected to the sink. Such attenuatorsare simple but because of the discontinuity introduced in the microwavepath by the diode, much of the energy is undesirably reflected backtoward the input port connected to the source.

In order to provide a shunt attenuator which is highly absorptive, i.e.,the reflective components from the diodes do not return to the source, apair of 90 hybrids are connected in tandem with shunt diodes placed atthe pair of tandem connections. One port of each hybrid is terminatedwith a resistive load, while the remaining port of one hybrid is theinput port of the attenuator and the remaining port of the other hybrid,the output port. While such attenuators are truly absorptive, and canhandle large amounts of power, they,

are relatively large and have a limited bandwidth.

In order to solve the size and bandwidth problems there have come intoexistence attenuators using series-shunt combinations of diodes in thetransmission line. However, such attenuators have limited power handlingcapabilities because some of the diodes are in series with the microwavesignal conductor of the transmission line. In addition, there is addedcomplexity because complicated diode biasing schemes are re;

quired.

It is, accordingly, an object of the invention to provide an improvedmicrowave signal attenuator.

It is another object of the invention to provide such an attenuatorwhich is about as large as the series-shunt attenuators but is easier tomanufacture than those attenuators and can handle considerably morepower.

It is a further object of the invention to provide an improved microwavesignal attenuator which requires a simple biasing scheme and which doesnot apply bias via the main portion of the transmission line.

Briefly, the invention contemplates a microwave signal conductor, suchas a transmission line, with an input port and an output port. Betweenthe two ports there are four points along the line spaced from eachother at odd multiples of quarter center-frequency wavelengths. (Bycenter-frequency wavelength is meant the wavelength for the microwavesignal at'its center or normal operating frequency.) Connected to thetwo inner points are shunt diodes for shunting microwave energy toground or other suitable reference potentials. Connected to the twoouter points are series circuits comprising an impedance and diode, theimpedance having a resistive component at the centerfrequency wavelengthsubstantially equal to the characteristic impedance of the microwavesignal conductor. A bias source is connected to the diodes in the seriescircuits so that current flows through these circuits and the other twodiodes to vary the shunt resistance of the diodes to control the amountof microwave energy flowing down the conductor to ground.

Other objects, the features and advantages of the invention will beapparent from the following detailed description of the invention whenread with the accompanying drawing whose sole FIGURE shows a microwavesystem utilizing the invention.

In the sole FIGURE, a microwave signal source-10 is connected via amicrowave sign-a l attenuator 12 to a microwave signal sink orutilization device 14.

The attenuator 12 comprises a length of transmission line 16 having oneend as an output port 20. DC. blocking capacitors can respectivelyconnect the ports 18 and 20 to the source 10 and sink l4. Positionedalong the line 16 are inner points 22 and 24 and outer points 26 and 28.All points are separated from each other by an odd multiple of quartercenter-frequency wavelengths. For the sake of compactness the idealseparation is one-quarter center-frequency wavelength. In addition,while only two inner points are shown, there can be more than two suchpoints. Connected between each inner point and ground is a PIN diode toprovide the shunting function. Thus, diode 30 is con- .nected to point22 and diode 32 to point 24.

Connected to each outer point is a series circuit. Typical seriescircuit 34 connected to point 26 comprises resistor 36, PIN diode 38 andreactive termination element 40. Resistor 36 is chosen to have aresistance substantially equal to the characteristic impedance of line16 while reactive termination element 40 is chosen to have a reactancewhich is equal in magniture to the reactance of the length of linebetween points 22 and 26 but to have the opposite reactance function Asuitable reactance terminal element can be a length of openendedtransmission line one-quarter ofa centerfrequ'enc y wavelength long withthe same characteristic impedance as line 16.

Connected to the junction of diode 38 and reactive termination element40, via a high impedance connection which can be a quarter wavelength ofline, is a bias source 42 in the form of an amplitude controlled DC.current generator. By DC. is meant a frequency much lower than thefrequency of the microwave signals. When the diodes are all polarized inthe same direction, as shown, current can flow from bias source 42, viadiodes 38 and 30 to ground. Thus, when diodes 30 and 30 have the sameshunt resistance (R,)/current (I) characteristic it should be apparentthat the resistances of these diodes are always equal to each other forany controlled current flow. Furthermore, since the resistance ofresistor 36 equals the characteristic impedance of the line 16 betweenthe points 22 and 26 then for the center frequency, the impedance seenat a point 26looking toward point 22 equals the impedance of circuit 34and any reflections because of diodes 30 are absorbed. Furthermore,because 'of the complementary action of reactive termination element 40on the reacthe component of the line 16 between points 22 and 26 isobtained for the at-' in that circuit and the shunting diode 32 withthese diodes polarized in the same direction as diodes 38 and 22. Thus,a single bias source can be used for all attenuation control.

There has thus been shown an improved microwave- 1. A microwave signalattenuator for attenuating signals having a given center-frequencycomprising a microwave signal conductor with a given characteristicimpedance and having an input port at one end and a signal output portat the other end, at least first and second diodes, said first diodeconnecting a first point on said microwave signal conductor to ground,said second diode connecting a second point on said microwave signalconductor to ground, said first and second points being separated by anodd number of quarter center-frequency wavelengths, a first circuitincluding an impedance having a resistive component with a value at saidcenter-frequency substantially equal to said characteristic impedanceand a third diode connected in series, one end of said first circuitbeing connected to a third point on said microwave signal conductorwhich is between said input port and said first point and displaced fromsaid first point by an odd number of quarter center-frequencywavelengths, a second circuit including an impedance having a resistivecomponent with a value at said center-frequency substantially equal tosaid characteristic impedance and a fourth diode connected in series,one end of said second circuit being connected to a fourth point on saidmicrowave signal conductor which is between said output port and saidsecond point and displaced from said second point by an odd number ofquarter centerfrequency wavelengths, and bias means for applying acurrent to said first circuit so that the same cuurrent flows throughsaid third and first diodes, and for applying a current to said secondcircuit so that the same current flows through said fourth and seconddiodes.

2. The microwave signal attenuator of claim 1 wherein said first circuitcomprises a first resistor, said third diode and a first reactiveimpedance connected in series and said second circuit comprises a secondresistor, said fourth diode and a second reactive impedance connected inseries, said resistors having values equal to the characteristicimpedance of said microwave signal conductor.

3. The microwave signal attenuator of claim 2 wherein said first andsecond reactive impedances have values such that they are equal inmagnitude and opposite in reactance function to the impedance betweensaid third-and-first points and said fourth-and-second points,respectively.

4. The microwave signal attenuator of claim 3 wherein each of said firstand second impedances is an open-ended transmission line having acharacteristic impedance substantially equal to the characteristicimpedance of said microwave signal and a length equal to an odd numberof quarter center wavelengths.

5. The microwave signal attenuator of claim 3 wherein an end of each ofsaid first and second resistors is connected to said third and fourthpoints, respec tively, and said bias means is connected to the junctionof said third diode and said first impedance and the junction of saidfourth diode and said second impedance.

6, The microwave signal attenuator of claim 3 wherein all of said diodeshave substantially the same shunt resistance-current characteristic andare polarized in the same direction to pass current from said bias meansto ground whereby equal currents flow through all of said diodes so thatall of said diodes have the same resistance at the center frequency.

7. The microwave signal attenuator of claim 6 wherein an end of each ofsaid first and second resistors is connected to said third and fourthpoints, respectively, and said bias means is connected to the junctionof said third diode and said first impedance and the junction of saidfourth diode and said second impedance

1. A microwave signal attenuator for attenuating signals having a givencenter-frequency comprising a microwave signal conductor with a givencharacteristic impedance and having an input port at one end and asignal output port at the other end, at least first and second diodes,said first diode connecting a first point on said microwave signalconductor to ground, said second diode connecting a second point on saidmicrowave signal conductor to ground, said first and second points beingseparated by an odd number of quarter center-frequency wavelengths, afirst circuit including an impedance having a resistive component with avalue at said center-frequency substantially equal to saidcharacteristic impedance and a third diode connected in series, one endof said first circuit being connected to a third point on said microwavesignal conductor which is between said input port and said first pointand displaced from said first point by an odd number of quartercenter-frequency wavelengths, a second circuit including an impedancehaving a resistive component with a value at said center-frequencysubstantially equal to said characteristic impedance and a fourth diodeconnected in series, one end of said second circuit being connected to afourth point on said microwave signal conductor which is between saidoutput port and said second point and displaced from said second pointby an odd number of quarter center-frequency wavelengths, and bias meansfor applying a current to said first circuit so that the same cuurrentflows through said third and first diodes, and for applying a current tosaid second circuit so that the same current flows through said fourthand second diodes.
 2. The microwave signal attenuator of claim 1 whereinsaid first circuit comprises a first resistor, said third diode and afirst reactive impedance connected in series and said second circuitcomprises a second resistor, said fourth diode and a second reactiveimpedance connected in series, said resistors having values equal to thecharacteristic impedance of said microwave signal conductor.
 3. Themicrowave signal attenuator of claim 2 wherein said first and secondreactive impedances have values such that they are equal in magnitudeand opposite in reactance function to the impedance between saidthird-and-first points and said fourth-and-second points, respectively.4. The microwave signal attenuator of claim 3 wherein each of said firstand second impedances is an open-ended transmission line having acharacteristic impedance substantially equal to the characteristicimpedance of said microwave signal and a length equal to an odd numberof quarter center wavelengths.
 5. The microwave signal attenuator ofclaim 3 wherein an end of each of said first and second resistors isconnected to said third and fourth points, respectively, and said biasmeans is connected to the junction of said third diode and said firstimpedance and the junction of said fourth diode and said secondimpedance.
 6. The microwave signal attenuator of claim 3 wherein all ofsaid diodes have substantially the same shunt resistance-currentcharacteristic and are polarized in the same direction to pass currentfrom said bias means to ground whereby equal currents flow through allof said diodes so that all of said diodes have the same resistance atthe center frequency.
 7. The microwave signal attenuator of claim 6wherein an end of each of said first and second resistors is connectedto said third and fourth points, respectively, and said bias means isconnected to the junction of said third diode and said first impedanceand the junction of said fourth diode and said second impedance.